This invention relates to determining formation continuity and, more particularly, relates to determining the continuity of a subsurface formation or layer penetrated by an existing borehole while drilling a second borehole adjacent the existing borehole, through that same subsurface formation.
Determining the continuity of a subsurface geologic layer penetrated by two adjacent boreholes is important to the oil and gas industry. In exploration, for example, such subsurface layer continuity information may be used to (1) construct a geologic model of the subsurface area containing the layer; (2) correlate observed formation properties between adjacent boreholes; and (3) determine if there are faults present in the layer. In production, for example, such continuity information may be used to (1) indicate the continuity of a reservoir layer between adjacent boreholes, and (2) indicate the presence of permeability barriers in such a reservoir layer.
U.S. Pat. No. 5,005,159 issued on Apr. 2, 1991 by Christine E. Krohn entitled "Continuity Logging Using Differenced Signal Detection" discloses a method for determining the continuity of a subsurface lithographic layer which is penetrated by two adjacent boreholes; this patent is incorporated herein by reference. This patent employs a seismic source lowered into a first borehole to a depth corresponding to the depth of the layer, or adjacent the layer, whose continuity is to be determined, while a receiver pair is simultaneously lowered into a second borehole to a depth corresponding to the depth of the layer, or adjacent that same layer. The recording of high amplitude signals at depths corresponding to the layer, i.e., within a layer, is an indication of a continuous low velocity layer extending through the subsurface between the two boreholes.
The problem with most downhole sources is that they are weak compared to surface sources. This is because the source must be relatively weak as compared to conventional surface seismic sources in order to avoid damage to the borehole wall. In addition, the operation of a downhole seismic source is expensive in a producing well (one producing hydrocarbon fluids) because the well must be shut in (cease producing fluids) during the use of the source.
A drill bit may be used as a downhole seismic source, as suggested in U.S. Pat. No. 4,718,048. The method described in this patent involves the convolution of signals recorded by conventional surface seismic detectors with a reference signal recorded on the drill string. The result is a seismogram as a function of time that is similar to that recorded by a conventional impulsive seismic source. This seismogram is similar to walk-away VSP and may be used to locate subsurface layers having appropriate reflection coefficients located adjacent the borehole. The seismogram is limited to frequencies of less than about 100 Hz because the reference signal does not include high frequency vibrations of the drill bit. Because of this frequency limitation this method is not capable of detecting thin (less than about 50 feet or so) reflective layers in the earth adjacent the borehole. Furthermore, this method cannot be used if there is excessive noise present in the frequency band of interest from the drilling rig.
These and other limitations and disadvantages of the prior art are overcome by the present invention, however, and an improved method is provided for determining the continuity of a subsurface geologic layer or formation that is penetrated by a preexisting borehole.